High efficiency removal of low density lipoprotein-cholesterol from whole blood

ABSTRACT

The present invention relates to the efficient removal of low density lipoprotein cholesterol complex (LDL-C) from whole blood. More specifically, it relates to a process for making a microporous plasmapheresis membrane having an immobilized affinity agent. The immobilized affinity agent is polyacrylic acid bound directly and/or through an interaction with silica and/or calcium chloride to a microporous hollow fiber membrane.

This is a divisional of U.S. Ser. No. 878,580, filed May 5, 1992, now U.S. Pat. No. 5,258,149, which is a continuation-in-part of U.S. Ser. No. 782,348, filed Oct. 31, 1991, now U.S. Pat. No. 5,236,644, which is a continuation-in-part of U.S. Ser. No. 618,791, filed Nov. 27, 1990, now U.S. Pat. No. 5,187,010.

TECHNICAL FIELD

The present invention relates to the efficient removal of low density lipoprotein cholesterol complex (LDL-C) from whole blood. More specifically, it relates to the use of an immobilized affinity agent on a microporous plasmapheresis membrane. The immobilized affinity agent is polyacrylic acid bound directly and/or through an interaction with amorphous silica and/or calcium chloride to a microporous hollow fiber membrane.

BACKGROUND

Atherosclerosis is the thickening and loss of elasticity in the inner walls of arteries, accompanied by the formation of small fatty modules on the artery walls and degeneration of the affected area. Atherosclerosis presented in the form of coronary heart disease and cerebrovascular diseases are major causes of morbidity and mortality in many industrial countries. Elevated plasma levels of low density lipoprotein-cholesterol complex (LDL-C) correlate with an increased risk for the development of atherosclerosis.

Patients at high risk for atherosclerosis are encouraged to make dietary changes in an attempt to control LDL-C levels. However, patient compliance is not always high and there is a large patient population which cannot control LDL-C levels merely through dietary modifications.

Drug therapy is also commonly used to try to lower LDL-C levels. While drug therapy is effective for many patients, there are still a large number of patients who are resistant to drug therapy or who suffer too many side effects to warrant its use.

In addition to dietary changes and drug therapy, attempts have been made to remove LDL-C directly from the plasma of patients through extracorporeal methods. These methods include plasma exchange, filtration based on molecular size, immunoadsorption, heparin precipitation and dextran sulfate adsorption. While these methods effectively remove LDL-C from plasma, they also remove varying quantities of desirable plasma components. The plasma exchange method removes all plasma and replaces the volume with plasma or albumin replacement solutions. All valuable plasma components, such as high density lipoprotein (HDL), and proteins such as albumin, IgG and clotting factors are removed in addition to the LDL-C. The other methods, while better than plasma exchange, have varying degrees of specificity for only LDL-C. With filtration based on molecular size, there is considerable loss of proteins with molecular weights greater than 250-400 kD. Immunoabsorption is specific for LDL-C only, but its efficiency for removal of LDL-C is not as great as other methods. Heparin precipitation and dextran sulfate adsorption remove LDL-C, but a loss of 20-40% of HDL is generally expected; also the adsorbing capacities are fairly low. Since HDL plays an important role in reducing a patient's risk for atherosclerosis, a method which eliminates or minimizes the loss of HDL is highly desirable.

Previous filtration methods have also utilized carriers, such as agarose beads, which lack mechanical strength, and as a result are difficult to handle and operate. When fluid is passed through these carriers, there is a high probability of blockage. Additionally, these carriers may be destroyed by sterilization techniques. These carriers might also leach materials into the patient fluid.

Polyacrylate has been tested as a sorbent for lipoproteins from human plasma (Thies et al., Artificial Organs (1988) 12(4):320-324). Negligible loss of HDL and plasma proteins was shown with this absorbent. Polyacrylate has been attached to cellulosic beads through amide linkages. While the preparation was useful, it was not optimal for the treatment of whole blood. As mentioned previously, cellulosic beads do not have good mechanical strength, block easily, and are not easily sterilized.

Kuroda et al. (EP 0143369) describe a porous adsorbent for absorbing low density lipoproteins having a silanol group and a synthetic polyanion linked with the surface. To prevent clogging, the porosity of the adsorbent must be distributed over a broad diameter range. By contrast, the microporous membrane of the present invention has uniform pore diameters. Murakami (Japanese P. A. 01-229878) describes porous polyester fibers coated with methacrylic acid which are useful to remove bilirubin or LDL from body fluids. Sterilization of polyester fibers can be problematic. Kuroda et al. (Japanese P. A. 63-232845) describe an absorbent material having on its surface a synthetic linear polymer which has both a carboxyl group and sulfate or sulfonate groups.

To date, the majority of extracorporeal methods for the removal of LDL-C have involved two separate steps. First, the blood must be separated into cellular components and plasma components. This is usually done through centrifugation or filtration. Second, the plasma is treated to remove LDL-C. Finally, the treated plasma and cellular components are returned to the patient. The procedures are both time consuming and require a great deal of handling of blood products, which leads to increased potential for infections. Also these methods require high extracorporeal volumes, which can be detrimental to the patient.

Methods involving a closed system which are relatively rapid, efficient, require limited handling of blood and reduce extracorporeal volumes are highly desirable.

SUMMARY OF THE INVENTION

The present invention provides an improved process for preparing a membrane capable of removing low density lipoprotein cholesterol complex (LDL-C) directly from whole blood. An immobilized affinity agent is integral to the microporous plasmapheresis membrane. LDL-C removal is achieved during the plasmapheresis process in a single step. The immobilized affinity agent is polyacrylic acid bound directly and/or through an interaction with silica and/or calcium chloride to a microporous polysulfone hollow fiber membrane.

In one aspect, the process for immobilizing polyacrylic acid to the hollow fiber membrane is conducted at an acidic pH. Under acidic conditions undesirable side products, such as calcium carbonate, does not form as they do under basic conditions. The product formed by this process and its performance are superior to products manufactured under other conditions.

In another aspect, this process provides membranes wherein unincorporated silica is substantially removed from the final product. Silica acts as a pore former and viscosifier in membrane formation. However, once the initial membrane is formed, the presence of silica, especially silica not incorporated into the membrane network, is not necessary. Residual silica can be removed by treating the membrane under basic conditions.

Another aspect of this invention is an improved process for drying the membrane. The membrane is more easily dried from basic pH solutions. The membrane preferably is returned to a basic pH before drying. The wettability of the membrane is further improved by the presence of a simple salt and non-ionic surfactant in the drying solution.

In one aspect, the invention relates to the effective and highly specific removal of LDL-C from the plasma portion of whole blood. The invention removes negligible amounts of HDL or other blood proteins.

In another aspect the invention is superior to prior extracorporeal methods in that whole blood passes through one device where it is simultaneously separated into plasma and cellular components, the LDL-C is removed from the plasma, and the treated plasma and cellular components are returned to the patient. The process is as rapid as conventional treatments and requires a minimum amount of blood handling.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGURE 1 is a schematic diagram indicating the action of the device of the invention.

DETAILED DESCRIPTION OF THE INVENTION

A membrane has been discovered which has properties that are advantageous for the removal of the complex of low density lipoprotein and cholesterol (LDL-C) from whole blood or plasma. The polysulfone hollow fiber membrane has polyacrylic acid immobilized on its surface. The membrane has desirable mechanical and specificity characteristics for its intended purpose of LDL-C removal. The membrane can also be sterilized by autoclaving techniques.

The Membrane

The membranes of this invention are polysulfone-based polymeric compositions. Polysulfones are a known class of polymers which have been used to form various types of membranes. Polysulfone membranes are of a substantially non-flexible physical form. "Polysulfone", "polyarylsulfone", "polyether sulfone", and "polyarylether sulfone" are each intended to define a polymeric material having a combination of sulfone groups, aryl groups, and ether groups in the polymer chain and which may also contain alkylene groups therein. Polysulfone (PS) polymers are available in a variety of grades with respect to molecular weight, additives, etc. High molecular weight polysulfones may be preferred for preparation of membranes with additional strength. UDEL® P-1700, and UDEL® 3500 polysulfone polymers (Amoco Performance Products Inc.) are suitable. Other suitable commercially available polysulfones are under the tradenames of ASTREL® (3M) VICTREX® (ICI) and RADEL® (Amoco). Polysulfone is used as the primary polymeric component of the membrane because of such beneficial characteristics as thermal stability, resistance to acid, alkali and salt solutions, high mechanical strength, etc.

The polysulfones found useful as membrane components of the present invention are polyaryl ether sulfones. The polysulfone can be viewed as having recurring units which is shown below: ##STR1## where the SO₂ group may be in the ortho, meta or para position on the ring and where R represents ##STR2## wherein n is an integer of 0 to 3 (preferably 0 or 1) and each R' independently is selected from hydrogen or a C₁ -C₃ alkyl, preferably methyl. The above polyarylether sulfones may be used as homopolymers or as copolymers of the polymeric groups described above where R is selected from more than one of the groups described hereinabove. Further, the above polyarylether sulfones may be formed into copolymers with polysulfone groups which are void of ether groups therein such as: ##STR3## and the like. The homopolymers and copolymers described above can be used as the sole polymeric component or mixtures or blends of the homopolymers and/or copolymers can be used as the membrane component. The formation of blends provides polymeric component which can have customized properties. For example, it is known that increase in ether oxygen and/or alkylene groups in the subject polymers provides decrease in the soften temperature of the polymeric component and, therefore, aids in providing a composition which can be processed at a designed temperature. The subject polysulfones can be prepared by known manners.

The polysulfones used herein should have a weight average molecular weight of from about 20,000 to about 200,000, preferably at least about 50,000 to about 150,000. The polymer Tg will be dependent upon the structure of the polymer as described above and can be determined by one skilled in the art by conventional analytical means.

The subject polysulfones have benzylic hydrogens which can be independently substituted by non-dissociative groups, such as alkyl (preferably C₁ -C₃ alkyl) or halogen (preferably chlorine) or by a dissociative group, such as sulfonic or carboxylic acid group. Each of the aryl groups may be unsubstituted or substituted with one or more of particular groups described above or may be substituted by different groups on a single aryl group or each on different aryl groups.

Other polymers or prepolymers can be used in combination with the polysulfone polymer, if desired, to impart various characteristics to the membrane product. Polyethylene glycol (PEG), polyvinyl pyrrolidone (PVP) or any of a variety of polyurethane prepolymers may be used with the polysulfone to prepare these membranes. Polymers or prepolymers are added to the polysulfone polymer in order to modify the structure and surface characteristics of the polysulfone membrane. The additional polymer or prepolymer becomes an integral part of the membrane structure.

A. The Casting Solution

The casting solution is a multicomponent solution comprising polymeric and solvent components. The primary polymeric component will be the polysulfone polymer. The polymeric component would, of course, also comprise any other polymer or prepolymer which is used together with the PS polymer to form the membranes. Where reference is made to the polysulfone solution or casting solution, it is intended to include all polymeric components. That is, it will include the polysulfone polymer and, where appropriate, it also will include a selected additional polymer or prepolymer as described above.

The solvent component of the casting solution must be one in which polysulfone (as well as any other polymer or prepolymer used) is soluble. The polysulfone polymer is soluble in various solvents, such as 4-butyrolactone, N-methylpyrrolidone (N-MP), dimethylformamide (DMF), N,N-dimethylacetamide (DMA), cyclohexanone, and chloroform. 4-Butyrolactone is the preferred solvent.

At least about 8.0 wt. % and up to about 35.0 wt. % polysulfone in solvent should be used, preferably about 8.0 to about 22.0 wt. %. Above 35 wt. %, it will be difficult or impossible to dissolve the polysulfone in the solvent. Below about 8%, precipitation will be too slow for formation of hollow fibers, and the fibers are too fragile to handle practically. Up to about 20.0 wt. % of a second polymeric component, that is, one or more of the polymers or prepolymers described above, can be added to the PS solution.

The casting solution can also contain silica. Silica can be present in amounts of about 0.1 to about 10% wt/wt, preferably about 5%. The silica does not dissolve in the casting solution, but rather forms a slurry. The silica aids in the immobilization of polyacrylic acid to the membrane during the next step of processing. Silica acts as a pore former and viscosifier to achieve a microporous structure with a nominal pore size of about 0.4 micron to about 0.65 micron. The casting solution can also contain polyacrylic acid (PAA). PAA can be present in amounts of about 0.01 to about 2% wt/wt, preferably about 0.5-1%.

B. Precipitation Solution

The precipitation or coagulation mechanism of membrane formation is affected by the composition of the precipitation solution as well as that of the casting solution, and the composition of these two solutions are interdependent. In this disclosure, the terms "precipitation solution", "coagulation solution," "quench solution," and "quench bath" are used interchangeably to refer to the solution in which the membrane is formed. For formation of hollow fiber membranes, both an outer and a center precipitation or quench solution will be employed. The solvent content of the precipitation solution controls the rate at which the solvent comes out of the casting solution. In turn, this controls the rate of increase of the polymer concentration to the point at which the polymeric component precipitates out of the casting solution to form the membrane. The same solvent usually is used in the casting solution and the precipitation solution. 4-butyrolactone and blends of 4-butyrolactone and N-methylpyrrolidone are the preferred solvents. Other solvents are discussed above with regard to casting solutions.

A non-solvent is often used in the precipitation solution in order to precipitate the polymer from the casting solution, thus causing formation of the membrane. For practical and economical purposes, it is preferred to use water as the non-solvent component of the precipitation solution. However, other non-solvents such as methanol, ethanol, propanol, butanol, ethylene glycol, acetone, methyl ethyl ketone, or the like, can be used instead of water, particularly when the solvent is water-immiscible. Alternatively, water and one or more other non-solvents can be used together.

In utilizing the method of this invention to prepare hollow fiber membranes, the precipitation solution used for the outer quench bath may be different from that used for the center quench fluid. In the preferred embodiment of this invention, the outer precipitation solution is water, and the center precipitation solution is 4-butyrolactone. Other solvents and non-solvents can be used as described above. In hollow fiber production, the center quench and outer quench are different phenomena. At center quench, a small volume of solution is used, which is almost in a static mode as compared with the casting solution. Conversely, the outer quench bath is present in large volumes and in a dynamic mode.

C. The Hollow Fiber Spinning Conditions

In preparing the hollow fiber membranes of this invention, a liquid-liquid or wet spinning process is used similar to that described in U.S. Pat. No. 4,970,030. That is, the casting solution is fed through an extrusion die (spinnerette) directly into a precipitation bath, while simultaneously introducing the center quench fluid through the central aperture of the spinnerette to mechanically maintain the hollow center hole of the fiber. The fiber is fabricated and simultaneously quenched as it is drawn through the precipitation bath. By using this wet-spinning process, fibers with homogeneous pore structure and membrane morphology are produced.

One of the key factors in preparation of the hollow fiber membranes of this invention is use of the wet spinning process; that is, spinning the casting solution under water. In addition, selection of appropriate solutions for the inner and outer precipitation baths is important, as is the appropriate drawing or spinning rate of the fiber as it is formed. The presence of the center quench fluid also allows for simultaneous polymer precipitation from both the inner and outer surfaces of the fiber. The spinning rate is adjusted to allow for exchange of components between the casting and precipitation solutions. The solvent is leached out of the casting solution and is replaced by the non-solvent from the precipitation solution. As a consequence, polymer precipitation occurs, leading to formation of the membrane.

Too rapid a drawing rate will cause breakage due to insufficient membrane formation to maintain membrane integrity or will cause elongation or deformation of the pores. Conversely, too slow a drawing rate will cause defects resulting from excessive pressure by the center quench solution, which may cause blow-outs in the fiber structure; also, non-circular fibers are produced. The preferred drawing rate will depend in part on the casting solution viscosity and temperature and in part on the factors described below. However, the drawing rate typically will be in the range of about 3.0 to about 30.0 feet per minute, preferably about 7.0 to about 15.0 feet per minute, and will produce round fibers.

The precise spinning conditions are adjusted in order to yield hollow fibers meeting the desired physical requirements of inner diameter and wall thickness. Centering of the central aperture of the spinnette is required in order to achieve a fiber having a uniform wall thickness. Any spinnerette suitable for the preparation of hollow fiber membranes may be used to prepare the membranes of this invention, however, quartz or glass spinnerettes are preferred in order to achieve the small inside diameters required of the hollow fibers of the invention. The spinning conditions left to be adjusted are the flow rate and pressure of the casting solution and the flow rate and pressure of the center quench fluid. These adjustments are well within the knowledge and ability of one of ordinary skill in this art. The preferred temperature for the casting solution will be in the range of ambient temperatures, although higher temperatures, e.g., up to about 70° C., may be employed to reduce the viscosity of the casting solution.

The dimensional and porosity characteristics of the membranes of this invention are such that LDL-C can pass through the fiber wall but most blood cells do not. Hemolysis occurs if numerous blood cells pass through the fibers, which is highly undesirable. However, passage of a small number of red blood cells through the fiber is acceptable. Generally speaking, membranes can be prepared which possess a pore diameter of between about 0.1 microns to about 0.7 microns, preferably between 0.4 and 0.65 microns. The inner diameter of the hollow fibers can range from about 150 to about 400 microns, PG,15 preferably about 325 microns. The wall thickness can range from about ten to several hundred microns, preferably about 75 to about 100 microns.

D. Silica Removal

Membranes which have been prepared from a casting solution containing silica are optionally treated to remove residual silica. Silica which is not an integral part of the membrane network and is exposed to the bulk solution can be removed by treating the membrane in a strong basic solution. The basic solution can be any basic conditions, preferably 0.3N to 2.5N sodium hydroxide, most preferably 1.0N to about 2.0N sodium hydroxide. The membrane is generally treated with the basic solution for greater than 5 hours at room temperature. Fibers with silica are not microporous until the fibers are treated in the base to remove the bulk of the silica. The basic solution also aids in endotoxin removal. After this basic treatment, the membrane can optionally be treated with an acidic solution (i.e., approximately 0.1N HCl) to further aid in endotoxin removal prior to polyacrylic acid immobilization.

E. Polyacrylic Acid Immobilization

Polyacrylic acid (PAA) is a selective affinity agent for LDL-C. The presence of PAA on the surface of the PS hollow fiber membrane enables the effective removal of LDL-C from the plasma components of whole blood. Polyacrylic acid is immobilized on the surface of the fiber walls when the fibers are heated under pressure, preferably by autoclaving, for about 20 to about 40 minutes at about 122° to about 130° C. in an acidic PAA solution. In a preferred embodiment, the fibers are bathed in a PAA-containing solution and degassed under vacuum prior to the heat immobilization step. PAA is present in the PAA-containing solution in amounts of about 0.01 to about 3.0% wt/wt, preferably about 0.5-2.0%. The acidic conditions fall in the pH range of about pH 1.5 to about pH 5.5, usually about pH 2.85. This is a very simple and inexpensive means for anchoring PAA onto the surface of porous membranes for use as an affinity agent to effectively bind LDL-C. The acidic conditions prevent the formation of undesirable side products such as calcium carbonate and silica-carbonate aggregates which can hinder the performance of the membrane. The membranes formed by this process have improved binding of LDL-C in the range of 10-12 mg LDL-C per ml of fiber wall volume.

Without wishing to be bound by any theory, it is believed that the vacuum degassing step followed by the autoclaving process allows all internal surfaces to be wet by the PAA solution. This enables the PAA to be immobilized on both the outer and inner surface of the PS hollow fiber membrane. The membrane is more effective at removing LDL-C when the vacuum degassing step is performed.

During the autoclaving step, PAA can be immobilized directly to the PS hollow fiber membrane or it can be immobilized indirectly through interactions with silica which may be embedded in the PS hollow fiber membrane. Greater amounts of PAA are immobilized to the membrane when silica is incorporated than without. While the actual nature of the interaction between PAA and silica is unknown, it is clear that addition of silica to the casting solution enhances the quantity of PAA bound to the membrane. This step also causes the fibers to be annealed and remain unaffected by subsequent autoclave steps.

Calcium chloride can also be added in or prior to this first autoclaving step to increase again the amount of PAA immobilized to the membrane, presumably by increasing the number of binding sites. The actual nature of the interaction between PAA and calcium chloride is believed to be complexation. It is clear that calcium chloride enhances the quantity of PAA bound to the membrane. Calcium chloride is added to the first autoclave solution in an amount of 0.01 to 3% wt/wt, preferably about 0.4%.

E. Sterilization/Cleaning

The membrane of the invention is treated in a manner to ensure that it is sterile, the fibers are annealed, and also that no trace of residual solvent is present in the final membrane to reduce any chance of solvent or unsterile products leaching into the patient. For sterilization/cleaning the membrane is autoclaved a second time for about 20 to about 40 minutes at 120° to 130° C. in deionized water. The membrane can be optionally vacuum degassed prior to this autoclave step also. The membrane is washed again in water or a basic solution, such as 10 mM NaHCO₃ and soaked overnight in a water bath at ambient temperature containing about 5 to about 20% glycerine. It may also be desirable to have a simple salt (i.e., NaCl) and about 0.001% to about 0.1%, preferably about 0.01% of a non-ionic surfactant (i.e., TWEEN®20 or TWEEN®80). This sterilization/cleaning process removes residual amounts of solvent and non-immobilized PAA. Unbound calcium chloride is removed by chelation. It is important that all calcium chloride is bound or removed by chelation to ensure that the membrane is not hemolytic and does not cause complement activation.

It is important to note that, if the fibers are autoclaved first in water, then in PAA, calcium chloride, and base, less PAA is incorporated in the membrane.

F. Drying

The membrane is placed in a basic solution and dried. The basic solution should be in the pH range of about 7.5 to about 10.5, preferably about pH 8.5. In one embodiment, NaHCO₃ is added to the water autoclave sterilization solution. It may also be desirable to have a simple salt (i.e., NaCl and a surfactant (i.e., TWEEN® 20 or TWEEN® 80) in the drying solution. The salt and surfactant improve the wettability of the resulting membrane. Glycerin is also added at about 5% to about 20%. The fiber bundles are placed on adsorbent paper and allowed to dry exposed to room temperature air. Alternatively, the fibers can also be dried under vacuum at room temperature more quickly.

The Device

The membranes are dried, preferably at room temperature in air containing less than 50% relative humidity to remove excess water. The fibers are then placed in a housing, and both ends of the fiber are potted in place in the housing. The preferred housing is a FOCUS® 70 Fiber housing (National Medical Care, a division of W. R. Grace & Co. Conn.) which is packed to about 42%-55% packing density with about 1200-1600 fibers per housing. Any other convenient hollow fiber housings may be used.

Use

The membranes and the device of this invention are excellently suited for removal of LDL-C from whole blood or plasma. FIGURE 1 is a schematic representation of the mechanics involved in using the LDL-C removal device of the invention. Whole blood is removed from the patient, typically from a vascular access point in arm 10 using suitable blood removal apparatus 14. Some suitable apparati for blood removal include hypodermic needles, fistulas, subclavian catheters or other in-dwelling catheters. The blood passes from blood removal apparatus 14 into whole blood tubing 16 and is pumped via optional blood pump 18 into LDL-C removal device 28. As whole blood is pumped through the lumen of the hollow fiber membrane of LDL-C removal device 28, plasma is forced through the channels of the microporous fibers and separated from the cellular components of the blood. The plasma is treated in LDL-C removal device 28 exiting via plasma exit port 30. The remaining blood components (high hematocrit blood) passes down through the lumen of the membrane(s) and out exit port 34. The treated plasma is pumped via optional plasma pump 32 through plasma tubing 36 and is reunited with the high hemocrit blood at junction 44. The whole blood is then returned to the patient along with additional saline 38 added through saline tubing 40 at junction 46 as necessary via return tubing 42 to suitable blood return apparatus 12. The pressure is monitored by monitor 20 before blood enters LDL-C removal device 28, while blood is in LDL-C removal device 28 by monitor 24, and as blood exits LDL-C removal device 28 by monitor 22. Pressure can be adjusted as necessary using blood pump 18 and plasma pump 32.

Within the LDL-C removal device the action is as follows. The nominal pore size of the hollow fiber is such that it will reject or prevent the passage of blood cells through the membrane, yet permits the free passage of plasma and specifically the high molecular weight components such as LDL-C (2-6 million Daltons) through the membrane wall structure. As the plasma passes through the wall of the membrane, it comes into direct contact with the affinity agent PAA, and LDL-C is bound to the wall surface. The plasma which exits through the outer surface of the membrane contains less LDL-C. In a single step, the hollow fiber cartridge separates the plasma from the blood, removes the LDL-C from the plasma, and returns both plasma and blood components to the patients. Under normal operating condition for treatment of whole blood (flow rate (Q)_(Plasma) ≦0.35Q_(inlet) and transmembrane pressure (TMP)<50 mm Hg), the cartridge is saturated with LDL-C in about 20-40 minutes. The operating conditions for plasma only can include significantly higher TMP since there is no concern for blood cell hemolysis. The cartridge can be substantially regenerated with a 1.0M salt wash with high speed flow in either direction, but optimally in the reverse direction of the blood flow. This substantial regeneration represents about 85-95% of the original binding capacity restored.

In many of the devices of the prior art, an arterial/venous fistula must be implanted in the patient prior to treatment in order to achieve blood access to support the required higher flow rates for the devices. The access is often in the form of a subclavian catheter and the implant procedure is very invasive. The implant procedure carries certain risks with it as well, such as increased chance of blood clots. The device of the present invention does not require such high flow rates, and therefore conventional direct intravenous therapy type vascular access is possible. This procedure is much less invasive and has fewer risks associated with it. The flow rates of the device of this invention are optimal when plasma outlet flow is maintained at equal to or less than 20% of the blood inlet flow rate and when the pressure difference between the blood inlet and plasma outlet (TMP) is maintained at less or equal to 40 mm Hg. Back pressure is maintained on the plasma outlet flow to prevent hemolysis in accordance with standard procedures for plasmapheresis membranes.

The membranes and device of this invention dramatically reduce the amount of LDL-C from whole blood or plasma. A significant quick reduction in LDL-C levels is advantageous for some patients and cannot be obtained using drug or dietary regimens. The present device also drops LDL-C levels very selectively and effectively which is not necessarily the case for prior art devices. The invention further can facilitate plaque regression of atherosclerotic lesions insofar as reduction of circulating LDL-C levels permits.

This device is useful for reducing LDL-C in any number of increased cholesterol disorders. The primary candidates for use of the device of the invention include young individuals homozygous for familial hypercholesterolemia who have a family history of heart disease, patients with severe coronary artery disease that are non-operable, and all potential bypass candidates. The most significant and acute cholesterol disorder is hypercholesterolemia and treatment of this disorder is certainly applicable to the device of the invention.

EXAMPLES

The following examples are intended to illustrate but not to limit the invention. The following abbreviations have been used throughout in describing the invention.

    ______________________________________     dl        deciliter(s)     °C.               degrees centrigrade     Q         flow rate     g         gram(s)     HDL       high density lipoprotein     hr        hour(s)     kD        kilodalton(s)     l         liter(s)     LDL-C     low density lipoprotein cholesterol complex     m         meter(s)     ml        milliliter(s)     min       minute(s)     M         molar     %         percent     PAA       polyacrylic acid     PS        polysulfone     psi       pounds per square inch     rpm       rotations per minute     T.C.      total cholesterol     TMP       transmembrane pressure     ______________________________________

Example 1 Hollow Fiber Membrane Formation

A particular membrane of the invention having polyacrylic acid and silica bound to the polysulfone hollow fiber membrane is prepared as follows. Polysulfone, 210 g (UDELL® 1700, CAS #25135-51-7), was added to 1690 g of 4-butyrolactone (Kodak, CAS #96-48-0), in a glass jar with a sealable top containing a teflon (or other inert) liner. The mixture was rolled continuously on a roller mill for 48-72 hours at room temperature until the polymer was dissolved. To this solution of polysulfone in 4-butyrolactone was added 100 g of silica (SYLOX® 2, Davison Division of W. R. Grace & Co. Conn. The jar was resealed and rolled continuously on the roller mill for at least 16 hours at room temperature to disperse the silica particles. This gave a casting solution that was 10.5 wt % in Polysulfone, 5 wt % in SYLOX® 2 and 84.5 wt % in 4-butyrolactone.

The casting solution was then centrifuged at 2,000 rpm for 10 minutes to settle any poorly suspended silica particles. Next, the casting solution was pumped through a 40 micron stainless steel screen at 60 psi of pressure with dry nitrogen gas as the source of the driving pressure. After filtration the casting solution was de-gassed under mechanical vacuum at less than 10 mm Hg for at least 15 minutes and put in a stainless steel kettle that could be pressurized for delivery of casting suspensions to nozzle. No substantial solvent was lost during this degassing procedure due to the low volatility of the solvent. Under 60 psi of dry nitrogen gas, the casting solution was extruded through a glass nozzle within an orifice under the surface of a bath of deionized water. The core liquid of the spinnerette was 4-butyrolactone, driven by 80 psi dry nitrogen gas. The hollow fiber fabricated from the process during the under water spinning process was collected on a revolving wheel partially submerged under water. When the appropriate number of fibers were collected (800-1,200 revolutions), the fiber bundle was removed from the wheel, cut to chosen lengths, and soaked 16 hours at room temperature in deionized water.

Example 2 Polyacrylic Acid Immobilization

Polyacrylic acid (Case #9003-01-4) was immobilized on the fibers of Example 1 in the following process. Five (5) bundles of fibers of 13-inch length, each containing 1600 fibers, were placed in 2.5 liters of a solution of 1.0 N sodium hydroxide in a stainless steel tray, de-gassed by vacuum of 28 mmHg for at least 10 minutes, and allowed to soak 16 hours at room temperature.

The fibers were then rinsed with 1.75 liters of 0.5% polyacrylic acid in order to neutralize the caustic. The bundles were then placed in 2.5 liters of 0.5% polyacrylic acid (pH 2.85) and 0.4% calcium chloride, de-gassed as above, then autoclaved for 30 minutes at 130° C. at 30 psi. The fiber bundles were then rinsed with deionized water to remove excess solution of PAA and calcium ions and autoclaved again in 2.5 liters of deionized water for 30 minutes at 130° C. at 30 psi. The fibers were then removed from the autoclave solution and soaked 16 hours at room temperature in a bath containing 5% glycerin, 0.1 M sodium chloride, and 0.01 M sodium bicarbonate, pH=8.3.

After soaking in the glycerin bath, the fibers were removed and allowed to air dry for 24 hours at room temperature on absorbant paper. The dried fiber bundles were placed in the proper size device, and both ends were potted in place with a biomedical grace epoxy-resin system (Emerson & Cummings, Division of W. R. Grace & Co. Conn., (Cat #674A and 674B) as per instructions. The fiber device was now ready for testing after the excess fiber and potting compounds were trimmed from both ends. Once the device was tested to ensure the microporous membranes maintained pressure as expected, it was ready to be used for removal of LDL-C from plasma and/or whole blood.

Example 3 Device Testing

A hollow fiber device as prepared in Example 2 containing 1200 fibers with a surface area of 1356 cm² and total wall volume of 7.7 ml was perfused with plasma from a 100 ml reservoir of high cholesterol human plasma. The recirculation of high LDL-C plasma through the device was maintained at a flow rate of 58 ml/min giving a shear rate of 130 sec⁻¹ to achieve a steady plasma filtration rate through the walls of the fibers. Plasma samples were taken from the plasma exit port and filtrated at time 0, 30 minutes, and 60 minutes. The average transmembrane pressure (TMP) remained constant throughout the run at 100 mmHg. Plasma filtrate flux values were 5.3 1/hr/m² at 30 minutes and 4.9 1/hr/m² at 60 minutes. Total cholesterol assays were performed on the plasma reservoirs using the Kodak EKTACHEM® DT60 and nephelometry (Beckman Auto ICS Catalog Number 449310) to determine the level of the LDL-C associated protein apolipoprotein B.

The total cholesterol (T.C.) level was reduced from an initial value of 289 mg/dl to 175 mg/dl. The apolipoprotein B concentration was reduced form 173 mg/dl to 78 mg/dl. The total protein levels, also determined on the Kodak EKTACHEM® DT60, went from 7.8 gm/dlk to 7.0 gm/dl. The difference in the pre- and post-total cholesterol values was used to determine the amount of T.C. removed from the plasma reservoir and a drop of 39.4% was observed. This corresponds to a binding of 14.8 mg total cholesterol per ml. of fiber wall volume.

The principles, preferred embodiments and modes of operation of the present invention have been described in the foregoing specification. The invention which is intended to be protected herein, however, is not to be construed as limited to the particular forms disclosed, since these are to be regarded as illustrative rather than restrictive. Variations and changes may be made by those skilled in the art without departing from the spirit of the invention. 

We claim:
 1. A membrane which binds low density lipoprotein cholesterol prepared by a process comprising:(a) preparing a hollow fiber membrane from a casting solution comprising about 8 to about 22 weight % of a polysulfone polymer; (b) submerging said hollow fiber membrane in an acidic solution comprising polyacrylic acid and 0.1 to about 3 weight % calcium chloride and having a pH range of about 1.5 to about 5.5; (c) vacuum degassing said submerged hollow fiber membrane of step (b); (d) immobilizing polyacrylic acid to said hollow fiber membrane by heating under pressure the submerged vacuum degassed fibers of step (c); and (e) annealing the hollow fiber membrane of step (d) by heating under pressure in water.
 2. The membrane of claim 1 wherein the membrane is soaked in a basic solution between step (a) and step (b).
 3. The membrane of claim 1 which is prepared by further steps comprising:(f) adjusting the solution of step (e) to a pH in the range of 7.5 to 10.5; and (g) drying the membrane in the presence of a simple salt and a surfactant.
 4. A membrane which binds low density lipoprotein cholesterol prepared by a process comprising:(a) preparing a hollow fiber membrane from a casting solution comprising about 8 to about 22 weight % of a polysulfone polymer; (b) submerging said hollow fiber membrane in an acidic solution comprising polyacrylic acid and 0.1 to about 3 weight % calcium chloride and having a pH range of about 1.5 to about 5.5; (c) immobilizing polyacrylic acid to said hollow fiber membrane by heating under pressure the submerged fibers of step (b); (d) vacuum degassing said submerged hollow fiber membrane of step (c); and (e) annealing the hollow fiber membrane of step (d) by heating under pressure in water.
 5. The membrane of claim 4 wherein the membrane is soaked in basic solution between step (a) and step (b).
 6. The membrane of claim 4 which is prepared by further steps comprising:(f) adjusting the solution of step (e) to a pH in the range of 7.5 to 10.5; and (g) drying the membrane in the presence of a simple salt and a surfactant. 